1. Field of the Invention
The present invention generally relates to a nuclear reactor and, more particularly, to a control rod used in a pressurized water reactor.
2. Related Art
For better understanding of the present invention, background techniques thereof will first be described in some detail. A representative example of a fuel assembly employed in a pressurized water reactor is shown in FIG. 3 of the accompanying drawings. As is shown in the figure and also well known in the art, a fuel assembly 31 includes a plurality of control rod guide tubes 34 fixedly held at both ends by an upper nozzle 32 and a lower nozzle 33, respectively, a plurality of fuel rods 35, and a plurality of supporting lattices 36 through which the control rod guide tubes 34 and the fuel rods 35 are inserted, wherein individual control rods 42 constituting, for example, a control rod cluster 41, as shown typically in FIG. 4, are inserted into the control rod guide tubes 34 from the above or withdrawn therefrom for the purpose of adjusting or regulating the reactor power. As is well known in the art, the number and the disposition of the control rod guide tubes 34 and the fuel rods 35 differ in dependence on the type or species the fuel assembly 31.
Referring to FIG. 5, the control rod cluster 41 is composed of a spider 43 operatively coupled to a driving shaft of a control rod driving unit (not shown) and a plurality of vanes 44 mounted radially on the outer peripheral surface of the spider 43, wherein the control rods 42 are held vertically in the upright state by means of these fingers 45, respectively. Disposition of the fingers 45 and hence that of the control rods 42 corresponds to the disposition of the control rod guide tubes 34 in the fuel assembly 31. As can be seen in FIG. 6, each of the control rods 42 includes a cladding tube 51 formed of stainless steel and hermetically closed at both ends thereof by a top end plug 52 connected to the finger 45 as mentioned above and a bottom end plug 53. Accommodated within the cladding tube 51 is a rod-like neutron absorber 54 which is formed of a neutron absorbing material such as an Agxe2x80x94Inxe2x80x94Cd (silver-indium-cadmium) alloy or boron carbide or the like and which is pressed downwardly against the bottom end plug 53 by a hold-down spring 55 disposed within the cladding tube 51 at a top end portion thereof.
At this juncture, it is to be mentioned that when the control rod 42 of the structure described above is inserted into the control rod guide tube 34 of the fuel assembly 31 loaded in the reactor, the neutron absorber 54 disposed within the control rod cladding tube expands in the axial direction as well as in the radial direction under irradiation of neutrons. Furthermore, there is great likelihood that the soundness or integrity of the cladding tube 51 is impaired due to the irradiation. In the cladding tube 51 itself, the neutron irradiation does increase gradually from the bottom end thereof toward the top end. On the other hand, with regard to the neutron absorber 54, it is noted that a lower end portion 54a thereof among others undergoes noticeable expansion in both the axial and radial directions, as mentioned above. In that case, expansion of the neutron absorber 54 in the axial direction can be absorbed by contraction of the hold-down spring 55. Accordingly, the integrity of the cladding tube 51 can be protected against impairment due to the expansion of the neutron absorber in the axial direction. By contrast, expansion of the neutron absorber 54 in the radial direction can not be absorbed by the hold-down spring 55. For this reason, such arrangement has heretofore been adopted to allow the expansion of the neutron absorber in the radial direction so that the diameter d, of the lower portion 54a of the neutron absorber 54 is reduced over a length L in the axial direction as compared with a diameter d, of the other ordinary portion of the neutron absorber.
Now referring to FIGS. 7 and 8, description will be made in detail of a method of determining magnitude of the diameter reduction (d0xe2x88x92d1) in the lower portion 54a of the neutron absorber in the radial direction and the axial length L thereof (i.e., length of the reduced-diameter portion in the axial direction). First referring to FIG. 7 which is an exaggerated section of a control rod, clearance between the neutron absorber and the cladding tube is enlarged at the lower portion 54a of the neutron absorber due to the reduction of diameter when compared with the clearance at the upper portion of the neutron absorber. Consequently, the cladding tube 51 is more likely to undergo deformation due to irradiation-induced creep at the location corresponding to the reduced-diameter portion of the neutron absorber, and initially the section form of the cladding tube becomes flattened. Magnitude of such deformation or strain of the cladding tube has to be suppressed enough to fall within a range of elastic deformation in order to ensure insertability of the control rod into the control rod guide tube. In other words, the strain of the control rod guide tube must be held so as not to exceed a strain equivalent to the yield strength or yield capability of the material forming the cladding tube. Further, any decrease in volume of the neutron absorber as a whole due to the reduction of diameter must essentially exert no influence on the neutron absorbing capability. Under the circumstances, the diameter reduction is determined by taking into account the requirements mentioned above.
On the other hand, as the neutron irradiation of the control rod progresses, the neutron absorber expands gradually not only in the axial direction but also in the radial direction to be ultimately brought into contact with the cladding tube, whereby an internal pressure is applied to the cladding tube consequently, the diameter of the cladding tube increases, bringing about strain in the circumferential direction. Thus, the length of the reduced-diameter portion or the lower portion 54a of the neutron absorber is so determined that in the state where the cladding tube has undergone enough neutron irradiation to expand, the strain induced in the circumferential direction in the cladding tube portion which corresponds to the lower end position of the reduced-diameter portion is substantially equivalent to the strain induced in the cladding tube portion corresponding to the upper end position of the reduced-diameter portion (which may be considered as corresponding to the lower end position of the ordinary diameter portion of the neutron absorber). More specifically, since the neutron irradiation dose has a distribution profile such that the dose attenuates along the longitudinal axis of the control rod in the upward direction, the length of the reduced-diameter portion is determined so that difference in the expansion due to difference in the neutron irradiation dose between the lower end portion of the neutron absorber having the ordinary diameter and the diameter-reduced lower end portion of the neutron absorber is equivalent to the diameter reduction. In this conjunction, FIG. 8 illustrates graphically a relation between locations or positions of a cladding tube along the longitudinal axis thereof as viewed from the bottom end of the neutron absorber and strains induced in the cladding tube in the circumferential direction.
As is well known in the art, various types of control rods are available. In a typical conventional control rod, the length L, the diameters d0 and d1 mentioned previously have heretofore been selected, for example, such that d0≈8.7 mm, d1≈d0xe2x88x920.13 mm and that L≈300 mm.
However, because the control rod cluster 41 constituted by an assembly of the control rods 42 is driven stepwise by the control rod driving unit, shock produced upon stepwise driving of the control rod cluster 41 acts repetitionally on the reduced-diameter portion 54a of the neutron absorber 54 as a compressive load in the axial direction. As a result of this, the diameter of the reduced-diameter portion 54a of the neutron absorber 54 is caused to increase progressively, which in turn promotes the expansion of the reduced-diameter portion 54a of the neutron absorber 54 in the radial direction, whereby the portion of the cladding tube located in the vicinity of the bottom end plug 53 may be abraded due to contact with the control rod guide tube 34 of the fuel assembly 31. Thus, the thickness of the cladding tube at that portion can decrease to such an extent that the control rod 42 within the control rod guide tube 34 may be injured or the control rod 42 jams, giving rise to problems.
In light of the state of the art described above, it is an object of the present invention to provide a control rod of an improved structure which is capable of suppressing expansion of a reduced-diameter portion of a neutron absorber which is caused to occur in the radial direction under the influence of shock applied upon stepwise driving of a control rod cluster, to thereby ensure integrity of a cladding tube over an extended period.
In view of the above and other objects which will become apparent as the description proceeds, there is provided according to a general aspect of the present invention a control rod for a nuclear reactor, which includes a cladding tube closed hermetically at both ends thereof by means of a top end plug and a bottom end plug, respectively, a rod-like neutron absorber loaded into the cladding tube and having a reduced-diameter portion of a smaller diameter than the other portion of the neutron absorber, the reduced-diameter portion being disposed on the side of the bottom end plug, and a hold-down spring for pressing the neutron absorber downwardly against the bottom end plug. The control rod mentioned above is characterized in that a sleeve is disposed within an annular space defined between an outer peripheral surface of the reduced-diameter portion of the neutron absorber and an inner peripheral surface of the cladding tube. In this conjunction, material and dimensions of the sleeve should preferably be so selected as to ensure a sufficient strength capable of withstanding the expansion of the reduced-diameter portion in the radial direction.
In a preferred mode for carrying out the invention, a lower peripheral edge of the aforementioned other portion of the neutron absorber may be chamfered with an upper peripheral edge of the aforementioned sleeve being chamfered in a shape complementary to the chamfered shape of the lower peripheral edge of the above-mentioned other portion so that the chamfered portion of the sleeve can bear on the chamfered lower peripheral edge of the other portion of the neutron absorber.
In another preferred mode for carrying out the invention, the cladding tube and the sleeve may each be formed of a stainless steel. However, the cladding tube may be formed of a stainless steel, whereas the sleeve may be formed of a material having a smaller thermal expansion coefficient than the stainless steel.
In still another preferred mode for carrying out the invention, the sleeve may be formed in a cylindrical shape having a top closed, wherein the neutron absorber may be physically separated into the reduced-diameter portion and the other portion such that the closed top of the sleeve intervenes between the reduced-diameter portion and the other portion.
In yet another preferred mode for carrying out the invention, the sleeve may have an outer diameter which is substantially equal to the outer diameter of the afore mentioned other portion of the neutron absorber. Further, the bottom end plug may have an axial dimension increased by a predetermined value when compared with an axial dimension of a bottom plug of a conventional control rod having the same axial length as the control rod according to the invention. The predetermined value should preferably be smaller than about 15 mm.
The above and other objects, features and attendant advantages of the present invention will more easily be understood by reading the following description of the preferred embodiments thereof taken, only by way of example, in conjunction with the accompanying drawings.